MK2a
The signal lines on the serial connector are driven from the GPIO lines of the StrongArm, via an RS-232 driver (a Maxim MAX3241 chip), a thick-film resistor array (a Bourns 4420P chip) and some other minor components. Some of the lines perform other roles when the player is in DC mode (typically, when in a car).
Code:

---

      Serial     GPIO   SA   Other Uses

============================================

   1   DCD  [in]   27   51   Telephone Mute

   2   RD   [in]    -    -

   3   TD   [out]   -    -

   4   DTR  [out]  23   55   debug LED?, MK2 SIR endec speed sel 1

   5   GND

   6   DSR  [in]   21   59

   7   RTS  [out]  24   54   MK2 SIR endec speed sel 2

   8   CTS  [in]   22   56   Headlight Sense

   9   GND*

---

Notes
DTR is sometimes called DTS
DCD is normally called CD
SA is the pin on the StrongArm upon which the signal is present
*As far as I can tell, pin 9 (normally ring indicator - RI) is grounded.
I couldn't work out where pins 6 and 15 of the 4420P go, nor pins 8 and 15 of MAX3241. Pins 1 and 10 on one side of the 4420P and pins 11 and 20 on the other side seem to be connected to their respective grounds.
I am unclear on whether pins 2 and 3 should be swapped, see this thread.